In an electrophotographic process, to keep a developer charge amount constant is very importance in order to keep high print quality. However, when images having a high printing ratio are continuously printed, a toner corresponding to the amount of toner consumed by development is supplied from a cartridge in order to keep toner density in the developer constant. At this time, since a large amount of toner is supplied, a chance of contact with carrier is reduced. Consequently, a sufficient toner charge amount is not obtained, and there arises a problem such as white background fogging or deterioration in gradation reproducibility due to excessive development.
On the other hand, there is an idea to increase the chance of contact between toner and carrier by raising the rotation speed of a developer mixer. However, when the rotation speed of the developer mixer is excessively increased, the deterioration of the toner progresses. Thus, the agitation of the developer is required to be performed at a minimum necessary mixer rotation intensity and in a short time.